Series connected impedance compensating systems are generally known and used for the dynamic balancing of reactive voltages on a power transmission line in response to varying load demands. Traditionally, compensating circuits have often employed some form of series connected capacitor arrangement. It is also known to utilize active solid-state components as part of an overall series compensating system in electric power transmission networks. Such arrangements, often referred to as a Unified Power Flow Controller (UPFC) or an Active Power Line Conditioner (APLC), may employ voltage-source inverters to inject a voltage in series with the source such that the load voltage is of a desired magnitude and phase with respect to the source voltage. An example is of such a system is provided in U.S. Pat. No. 5,198,746 to Gyugyi et al., entitled "Transmission line Dynamic Impedance Compensation System".
Because the injected series voltage comprises only a small fraction of the total load voltage, series compensating systems are not very effective in controlling large current flows which can occur during fault conditions (e.g., due to lighting strikes or ground fault switching transients). In an electrical power transmission network (or "supply network") currents that flow during a short-circuit imposed on the network are predominantly governed by the source voltage and series line impedance of the network. During many fault conditions, such currents can potentially reach ten or twenty times the maximum rated current handling capacity of the power electronics used in conventional impedance compensating systems. It is desirable to protect the active power electronics of such systems against damage due to high current surges associated with fault conditions. One approach to providing such protection is demonstrated by U.S. Pat. No. 5,287,288 to Brennen et al. which discloses an active power line conditioner that employs a pair of opposing thyristors for forming a "crowbar switch" across the ac output of a transformer-coupled series voltage-source inverter (VSI). The crowbar switch provides a shorting path for shunting excess currents and thereby protects the VSI.
The Brennen et al. patent describes a dual thyristor crowbar switch triggered by a somewhat complicated rectifying bridge and Zener diode arrangement that senses overcurrents by detecting overvoltages at the ac side of the series inverter. Such an arrangement requires many components and inevitably increases the expense and decreases the overall reliability of the compensation system. The present invention provides an improved overcurrent sensing arrangement that monitors voltage fluctuations at a transformer coupling for the VSI or at the VSI, and provides an improved crowbar switch structure having few active components and simple to control. These improvements result in providing protection of power electronics for series compensating systems that is more reliable, lower in cost and particularly suitable for use in three-phase power distribution systems.
In one exemplary embodiment of the present invention (FIGS. 3 and 4), a conventional metal oxide varistor (MOV)--bypass breaker combination may be provided on the line side of a coupling series transformer (T). The coupling transformer includes a tertiary winding that is used to detect over-voltages in the line and is coupled to a current shunting thyristor "crowbar" switch (S.sub.CB). The series transformer includes a primary winding to which the VSI circuit is coupled and a secondary winding to which the line is connected. Alternating current from the VSI circuit flows through the primary winding and is picked-up by the secondary winding which provides the ac current to the line.
Each power phase leg of the VSI has three states with respect to the gating of the active portion, namely "high", "low", or "blocked." In the high state, the active device connected to the positive dc bus (source link) is gated "on", and the active device connected to the negative dc bus is gated "off." In the low state, the active device connected to the negative dc bus is gated "on" and the active device connected to the positive dc bus is gated "off." During these states inverter current will flow either in the active device that is gated to the "on" state or in the opposing diode. Conversely, in the blocked state both upper and lower active devices are gated off and current is forced to flow only through the diodes. During normal control, the inverter legs are switched between the high and low states to manage the dc voltage as desired. When outside forces cause the VSI voltage to exceed the rating of the active device, overcurrent protection is necessary to prevent damage to the VSI since its active solid-state devices generally cannot survive a turn-off event under excessive current.
The tertiary winding is between the primary and secondary windings of the transformer. If the tertiary winding is shunted by the crowbar switch, then the magnetic flux in the transformer is interrupted, and the VSI circuit is effectively isolated from the line by the transformer. A current sensor detects excessive current conditions at the tertiary winding and signals the crowbar switch to close the thyristors when an over-current condition is detected. The closed thyristors shunt the current in the tertiary winding and thereby isolate the VSI from the line. When the excessive current condition subsides then the thyristors automatically open to allow current to flow to the VSI.
Overvoltage protection is provided in one embodiment of the present invention by the third winding of the coupling transformer and a crowbar switch (S.sub.CB) coupled to the third winding. When an excessive voltage or current pulse the line (L.sub.line), an overvoltage condition in the coupling transformer at the third winding where it is detected by a voltage sensor coupled that third winding of the transformer. The sensor is associated with a crowbar switch circuit, and generates a signal to trigger a current shunting thyristor "crowbar" switch (S.sub.CB). The signal closes three thyristors (where one thyristor corresponds to each line (phase) of the line current) to shunt the current in the third winding of the transformer. The closed thyristors short out the third winding such that all current goes to this winding instead of the secondary winding associated with the VSI inverter. The transformer is a series transformer with the third winding between the windings coupling the line and the VSI circuit to the transformer. As the crowbar circuit shunts current in the third winding, the magnetic flux flow in the transformer stops which isolates the line from the VSI circuit.
Once the overcurrent pulse or condition subsides, the thyristors in the crowbar switch each automatically open when the phases of the line current next present a null current condition at the thyristor. When the thyristors open in the crowbar circuit, current flows again through the secondary winding which allows the transformer to coupled the VSI circuit to the line. Accordingly, the third winding and crowbar circuit protects the VSI circuit from voltage spikes and overcurrents in the line by shunting current to the third winding when such spikes and overcurrents are detected.
In accordance with the present invention, the improved crowbar switch circuitry utilizes pairs of diodes and thyristors for each leg (phase) of the line current. For overcurrent protection, the thyristors are operated by a control circuit that is triggered by a sensor detecting overcurrents at the tertiary winding. The crowbar circuit of the present invention is believed to be less expensive than the overcurrent protection circuit shown in the Brennen et al. patent that has anti-parallel thyristors for each phase of the line current. In addition, the crowbar switch does not require an external gate signal for activation. The switch is activated by a sensor that monitors the current on the third winding of the transformer. When the current increases beyond a threshold, the thyristors of the switch close to shunt current through the third winding and the switch. When the current drops below the threshold, the thyristors return to their normal open state.
In addition, because the crowbar switch is somewhat isolated from the VSI circuit by the third transformer winding, the voltage and current ratings for the crowbar circuit can be selected independently of the voltage and current ratings of the VSI circuit.